This invention relates to low modulus elastomers suitable for use as insulating and sealing materials in electrical products.
Elastomeric materials are available with a wide range of secant moduli (also referred to as M.sub.100 because it is measured at 100% elongation). At the high end are conventional rubbers, which have moduli in excess of 80 psi. At the low end are gels, sometimes referred to as gelatinous elastomers, having moduli below 2 psi.
A rubber can be the primary insulation in electrical equipment, but its relatively high modulus renders it insufficiently conformable to act as a water sealant, especially where equipment being insulated has a complex topography. To provide sealing, a sealant such as a mastic or adhesive may be interposed between the rubber primary insulation and the underlying equipment. To insulate electrical cable joints, terminations, and like equipment, a rubber can be fabricated as a heat-shrinkable article which is positioned over the equipment and then shrunk by heating, to provide a snug fit (albeit not necessarily water-tight fit in the absence of a sealant). Two disadvantages of a heat-shrinkable article are the need during manufacture for an expansion step and the need for the application of heat during the shrinking step. The expansion step introduces additional cost to the manufacturing process and the range-taking capability of a heat-shrinkable article is limited by the extent of expansion. The application of heat is undesirable as requiring an additional heating tool and as being inconvenient or hazardous at certain installation sites (cramped quarters, possible presence of flammable gases, etc.).
An alternative design employs a fairly high modulus rubber which is held out in an expanded shape by an underlying rigid hold-out element, typically having a spiral shape. After sliding the rubber over the area to be protected, the hold-out is pulled out, permitting the rubber to recover. Disadvantages of this design include the high modulus of the rubber and the possibility of tangling of the hold-out element during its pull-out.
Yet another design is a pre-molded splice case, in which molded insulation closely conforming to the dimensions of the equipment to be protected is slid over (with the aid of a lubricant if needed) over the equipment. Because of the rigidity of the molded material, this design has limited range-taking capability and may be difficult to install.
Conversely, a gel is highly conformable and possess good tackiness, making it an effective sealant. However, its low modulus precludes its use as a primary insulation. Generally, it is contained within an article (such as a connector), which provides structural support. Most gels are highly filled with a low molecular weight extending oil, which is can diffuse into a substrate with which it is in contact over time during service if a critical pressure is exceeded. Such exudation is undesirable. It is common in high voltage cable joints and terminations to have a semiconductive material ("semicon") present, to provide electrical stress grading. The exuded oil may migrate into the semicon or the cable jacket, adversely affecting their electrical properties or swelling them.